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Pointers. Introduction. Pointers Powerful, but difficult to master Simulate pass-by-reference Close relationship with arrays and strings. count. countPtr. count. 7. 7. Pointer Variable Declarations and Initialization. Pointer variables Contain memory addresses as values

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Introduction
Introduction

  • Pointers

    • Powerful, but difficult to master

    • Simulate pass-by-reference

    • Close relationship with arrays and strings


Pointer variable declarations and initialization

count

countPtr

count

7

7

Pointer Variable Declarations and Initialization

  • Pointer variables

    • Contain memory addresses as values

    • Normally, variable contains specific value (direct reference)

    • Pointers contain address of variable that has specific value (indirect reference)

  • Indirection

    • Referencing value through pointer


Pointer declaration
Pointer Declaration

- *indicates variable is pointer

<type> *<pointerName>;

  • Examples

    int*myPtr;

    - declares pointer to int, pointer of type int *

    int*myPtr1, *myPtr2;

    - Multiple pointers require multiple asterisks

    int* Ptr1, Ptr2;

    int*myPtr, var ;


Pointer initialization
Pointer Initialization

  • Can declare pointers to any data type

  • Pointer initialization

    • Initialized to 0, NULL, or address

      • 0 or NULL points to nothing

      • Examples

        int*Ptr1=0, *Ptr2=NULL ;


Pointer operators

yptr

y

y

5

yPtr

500000

600000

600000

5

address of y is value of yptr

Pointer Operators

  • & (address operator)

    • Returns memory address of its operand

    • Example

      int y = 5;int *yPtr;yPtr = &y; // yPtr gets address of y

    • yPtr “points to” y


Pointer operators1
Pointer Operators

  • * (indirection/dereferencing operator)

    • Returns synonym for object its pointer operand points to

    • *yPtr returns y (because yPtr points to y).

      *yptr = 9; // assigns 9 to y

  • * and & are inverses of each other


Example

* and & are inverses of each other

Example

1

2 // Using the & and * operators.

3 #include <iostream>

4

5 using std::cout;

6 using std::endl;

7

8 int main()

9 {

10 int a; // a is an integer

11 int *aPtr; // aPtr is a pointer to an integer

12

13 a = 7;

14 aPtr = &a; // aPtr assigned address of a

15

16 cout << "The address of a is " << &a

17 << "\nThe value of aPtr is " << aPtr;

18

19 cout << "\n\nThe value of a is " << a

20 << "\nThe value of *aPtr is " << *aPtr;

21

22 cout << "\n\nShowing that * and & are inverses of "

23 << "each other.\n&*aPtr = " << &*aPtr

24 << "\n*&aPtr = " << *&aPtr << endl;

25


Example1

* and & are inverses; same result when both applied to aPtr

Example

26 return0; // indicates successful termination

27

28 } // end main

The address of a is 0012FED4

The value of aPtr is 0012FED4

The value of a is 7

The value of *aPtr is 7

Showing that * and & are inverses of each other.

&*aPtr = 0012FED4

*&aPtr = 0012FED4


Calling functions by reference
Calling Functions by Reference

  • 3 ways to pass arguments to function

    • Pass-by-value

    • Pass-by-reference with reference arguments

    • Pass-by-reference with pointer arguments

  • return can return one value from function

  • Arguments passed to function using reference arguments

    • Modify original values of arguments

    • More than one value “returned”


Calling functions by reference1
Calling Functions by Reference

  • Pass-by-reference with pointer arguments

    • Simulate pass-by-reference

      • Use pointers and indirection operator

    • Pass address of argument using & operator

    • Arrays not passed with & because array name already pointer

    • * operator used as alias/nickname for variable inside of function


Example2

Pass number by value; result returned by cubeByValue

Example

1

2 // Cube a variable using pass-by-value.

3 #include <iostream>

4

5 using std::cout;

6 using std::endl;

7

8 intcubeByValue( int ); // prototype

9

10 int main()

11 {

12 int number = 5;

13

14 cout << "The original value of number is " << number;

15

16 // pass number by value to cubeByValue

17 number = cubeByValue( number );

18

19 cout << "\nThe new value of number is " << number << endl;

20

21 return0; // indicates successful termination

22

23 } // end main

24


Example3

Cubes and returns local variable n

cubeByValue receives parameter passed-by-value

Example

25 // calculate and return cube of integer argument

26 int cubeByValue( int n )

27 {

28 return n * n * n; // cube local variable n and return result

29

30 } // end function cubeByValue

The original value of number is 5

The new value of number is 125


Example4

Prototype indicates parameter is pointer to int

Apply address operator & to pass address of number to cubeByReference

cubeByReference modified variable number

Example

1

2 // Cube a variable using pass-by-reference

3 // with a pointer argument.

4 #include <iostream>

5

6 using std::cout;

7 using std::endl;

8

9 voidcubeByReference( int * ); // prototype

10

11 int main()

12 {

13 int number = 5;

14

15 cout << "The original value of number is " << number;

16

17 // pass address of number to cubeByReference

18 cubeByReference( &number );

19

20 cout << "\nThe new value of number is " << number << endl;

21

22 return0; // indicates successful termination

23

24 } // end main

25


Pointers

cubeByReference receives address of int variable,

i.e., pointer to an int

Modify and access int variable using indirection operator *

26 // calculate cube of *nPtr; modifies variable number in main

27 void cubeByReference( int *nPtr )

28 {

29 *nPtr = *nPtr * *nPtr * *nPtr; // cube *nPtr

30

31 } // end function cubeByReference

The original value of number is 5

The new value of number is 125


Pointer expressions and pointer arithmetic

location

3000

3004

3008

3012

3016

pointer variable vPtr

v[0]

v[1]

v[2]

v[4]

v[3]

Pointer Expressions and Pointer Arithmetic

  • Pointer arithmetic

    • Increment/decrement pointer (++ or --)

    • Add/subtract an integer to/from a pointer( + or += , - or -=)

    • Pointers may be subtracted from each other

    • Pointer arithmetic meaningless unless performed on pointer to array

  • 5 element int array on a machine using 4 byte ints

    • vPtr points to first element v[ 0 ], which is at location 3000

      vPtr = 3000

    • vPtr += 2; sets vPtr to 3008

      vPtr points to v[ 2 ]


Pointer expressions and pointer arithmetic1
Pointer Expressions and Pointer Arithmetic

  • Subtracting pointers

    • Returns number of elements between two addresses

      vPtr2 = v[ 2 ];vPtr = v[ 0 ];vPtr2 - vPtr == 2

  • Pointer assignment

    • Pointer can be assigned to another pointer if both of same type

    • If not same type, cast operator must be used


Pointer expressions and pointer arithmetic2
Pointer Expressions and Pointer Arithmetic

  • Pointer comparison

    • Use equality and relational operators

    • Comparisons meaningless unless pointers point to members of same array

    • Compare addresses stored in pointers

    • Example: could show that one pointer points to higher numbered element of array than other pointer

    • Common use to determine whether pointer is 0 (does not point to anything)


Relationship between pointers and arrays
Relationship Between Pointers and Arrays

  • Arrays and pointers closely related

    • Array name like constant pointer

    • Pointers can do array subscripting operations

  • Accessing array elements with pointers

    • Element b[ n ] can be accessed by *( bPtr + n )

      • Called pointer/offset notation

    • Addresses

      • &b[ 3 ] same as bPtr + 3

    • Array name can be treated as pointer

      • b[ 3 ] same as *( b + 3 )

    • Pointers can be subscripted (pointer/subscript notation)

      • bPtr[ 3 ] same as b[ 3 ]


Example5

Using array subscript notation.

Example

1

2 // Using subscripting and pointer notations with arrays.

3

4 #include <iostream>

5

6 using std::cout;

7 using std::endl;

8

9 int main()

10 {

11 int b[] = { 10, 20, 30, 40 };

12 int *bPtr = b; // set bPtr to point to array b

13

14 // output array b using array subscript notation

15 cout << "Array b printed with:\n"

16 << "Array subscript notation\n";

17

18 for ( inti = 0; i < 4; i++ )

19 cout << "b[" << i << "] = " << b[ i ] << '\n';

20

21 // output array b using the array name and

22 // pointer/offset notation

23 cout << "\nPointer/offset notation where "

24 << "the pointer is the array name\n";

25


Example6

Using array name and pointer/offset notation.

Using pointer subscript notation.

Using bPtr and pointer/offset notation.

Example

26 for ( int offset1 = 0; offset1 < 4; offset1++ )

27 cout << "*(b + " << offset1 << ") = "

28 << *( b + offset1 ) << '\n';

29

30 // output array b using bPtr and array subscript notation

31 cout << "\nPointer subscript notation\n";

32

33 for ( int j = 0; j < 4; j++ )

34 cout << "bPtr[" << j << "] = " << bPtr[ j ] << '\n';

35

36 cout << "\nPointer/offset notation\n";

37

38 // output array b using bPtr and pointer/offset notation

39 for ( int offset2 = 0; offset2 < 4; offset2++ )

40 cout << "*(bPtr + " << offset2 << ") = "

41 << *( bPtr + offset2 ) << '\n';

42

43 return0; // indicates successful termination

44

45 } // end main


Example7
Example

Array b printed with:

Array subscript notation

b[0] = 10

b[1] = 20

b[2] = 30

b[3] = 40

Pointer/offset notation where the pointer is the array name

*(b + 0) = 10

*(b + 1) = 20

*(b + 2) = 30

*(b + 3) = 40

Pointer subscript notation

bPtr[0] = 10

bPtr[1] = 20

bPtr[2] = 30

bPtr[3] = 40

Pointer/offset notation

*(bPtr + 0) = 10

*(bPtr + 1) = 20

*(bPtr + 2) = 30

*(bPtr + 3) = 40


Pointer based strings
Pointer-based strings

  • String as arrays of Characters

    char color[] = “Hello";

    char color[] = { ‘H', ‘e', ‘l', ‘l', ‘o', '\0' };

  • String as pointer to character

    char *colorPtr = "blue";

colorPtr

‘H’

‘e’

‘l

‘l’

‘o’

‘\0’

2

3

4

0

1

5


Example8
Example

1

2 // Copying a string using array notation

3 // and pointer notation.

4 #include <iostream>

5

6 using std::cout;

7 using std::endl;

8

9 void copy1( char *, const char * ); // prototype

10 void copy2( char *, const char * ); // prototype

11

12 int main()

13 {

14 char string1[ 10 ];

15 char *string2 = "Hello";

16 char string3[ 10 ];

17 char string4[] = "Good Bye";

18

19 copy1( string1, string2 );

20 cout << "string1 = " << string1 << endl;

21

22 copy2( string3, string4 );

23 cout << "string3 = " << string3 << endl;

24

25 return0; // indicates successful termination


Example9

Use array subscript notation to copy string in s2 to character array s1.

Use pointer notation to copy string in s2 to character array in s1.

Increment both pointers to point to next elements in corresponding arrays.

Example

26

27 } // end main

28

29 // copy s2 to s1 using array notation

30 void copy1( char *s1, const char *s2 )

31 {

32 for ( int i = 0; ( s1[ i ] = s2[ i ] ) != '\0'; i++ )

33 ; // do nothing in body

34

35 } // end function copy1

36

37 // copy s2 to s1 using pointer notation

38 void copy2( char *s1, constchar *s2 )

39 {

40 for ( ; ( *s1 = *s2 ) != '\0'; s1++, s2++ )

41 ; // do nothing in body

42

43 } // end function copy2

string1 = Hello

string3 = Good Bye


Arrays of pointers

’\0’

’\0’

’\0’

’\0’

’n’

’d’

’o’

’u’

’a’

’s’

’d’

’b’

’m’

’H’

’s’

’a’

’D’

’i’

’a’

’s’

’l’

’C’

’r’

’s’

’S’

’p’

’e’

’t’

’e’

suit[0]

suit[1]

suit[2]

suit[3]

Arrays of Pointers

  • Arrays can contain pointers

    • Commonly used to store array of strings

      char *suit[ 4 ] = {"Hearts", "Diamonds","Clubs", "Spades" };

    • Each element of suit points to char * (a string)

    • Array does not store strings, only pointers to strings

    • suit array has fixed size, but strings can be of any size